Improve validation of configured language options

src/ejabberd_config.erl

@@ -1449,7 +1449,7 @@ opt_type(hosts) ->
 	    [iolist_to_binary(H) || H <- L]
     end;
 opt_type(language) ->
-    fun iolist_to_binary/1;
+    fun xmpp_lang:check/1;
 opt_type(max_fsm_queue) ->
     fun (I) when is_integer(I), I > 0 -> I end;
 opt_type(default_db) ->

src/mod_muc.erl

@@ -988,7 +988,7 @@ mod_opt_type({default_room_options, presence_broadcast}) ->
 	      end, L)
     end;
 mod_opt_type({default_room_options, lang}) ->
-    fun iolist_to_binary/1.
+    fun xmpp_lang:check/1.
 
 mod_options(Host) ->
     [{access, all},